2way has it right tho. If the lower is drilled for, and will accept, the full auto sear, it doesn't matter if the full auto parts have been removed. The auto sear can be replaced in five minutes or less, and the other parts will drop right in.
I think this may be a question for the ATF to answer, as much as I hate to send anybody there for anything. My hunch is that they're going to say, "once a machine gun, always a machine gun"; but that's all it is, a hunch.[:D]